Hemoglobin oxygen saturation (OS) of blood is defined as the ratio of the oxyhemoglobin (HbO.sub.2) concentration to the total hemoglobin (Hb) concentration. One of the most common methods for measuring blood OS requires removal and analysis of a sample of the patient's blood. Analysis of an actual sample of blood is still considered the most accurate method for obtaining a reading of absolute blood oxygen saturation. However, this method is undesirable in cases where it is necessary to monitor blood oxygen saturation over long periods of time.
In many clinical situations, it is extremely important to be able to obtain continuous measurements of tissue oxygenation. While it is desirable to have an absolute measure of OS, it is often sufficient to measure relative changes in the blood oxygen saturation. For example, in the operating room, the physician is typically concerned only with significant changes in the patient's OS, and is less concerned with the measurement of absolute OS. In this situation, a noninvasive oximeter which is capable of detecting significant changes in the blood oxygen content would be especially useful.
It is well known that hemoglobin and oxyhemoglobin have different optical absorption spectra and that this difference in absorption spectra can be used as a basis for an optical oximeter. Most of the currently available oximeters using optical methods to determine blood oxygen saturation are based on transmission oximetry. These devices operate by transmitting light through an appendage such as a finger or an earlobe. By comparing the characteristics of the light transmitted into one side of the appendage with that detected on the opposite side, it is possible to compute oxygen concentrations. The main disadvantage of transmission oximetry is that it can only be used on portions of the body which are thin enough to allow passage of light.
There has been considerable interest in recent years in the development of an oximeter which is capable of using reflected light to measure blood oxygen saturation. A reflectance oximeter would be especially useful for measuring blood oxygen saturation in portions of the patient's body which are not well suited to transmission measurements. Experimental results suggest that it is possible to obtain accurate indications of blood oxygen content through the use of reflectance techniques.
A theoretical discussion of a basis for the design of a reflectance oximeter is contained in "Theory and Development of a Transcutaneous Reflectance Oximeter System for Noninvasive Measurements of Arterial Oxygen Saturation," by Yitzhak Mendelson (Published Doctoral Dissertation), No. 8329355, University Microfilms, Ann Arbor, Mi. (1983). A theoretical discussion of the optical properties of blood is found in "Optical Scattering in Blood," by Narayanan R. Pisharoty, (Published Doctoral Dissertation), No. 7124861, University Microfilms, Ann Arbor, Mi. (1971).
Numerous other works have disclosed theoretical approaches for analyzing the behavior of light in blood and other materials. The following is a brief list of some of the most relevant of these references: "New Contributions to the Optics of Intensely Light-Scattering Materials, Part 1," by Paul Kubelka, Journal of the Optical Society of America, Volume 38, No. 5, May 1948; "Optical Transmission and Reflection by Blood," by R. J. Zdrojkowski and N. R. Pisharoty, IEEE Transactions on Biomedical Engineering, Vol. BME-17, No. 2, April 1970; and "Optical Diffusion in Blood," by Curtis C. Johnson, IEEE Transactions on Biomedical Engineering, Vol. BME-17, No. 2, April 1970.
Various methods and apparati for utilizing the optical properties of blood to measure blood oxygen saturation have been shown in the patent literature. Representative devices for utilizing the transmission method of oximetry have been disclosed in U.S. Pat. Nos. 4,586,513; 4,446,871; 4,407,290; 4,226,554; 4,167,331; and 3,998,550. In addition, reflectance oximetry devices and techniques are shown generally in U.S. Pat. Nos. 4,447,150; 4,086,915; and 3,825,342.
Despite the advances shown in the above-mentioned references, the prior art is still lacking a satisfactory noninvasive reflectance oximeter which can be used to estimate blood oxygen saturation. In particular, there is a need for a noninvasive reflectance oximeter which can be quickly and easily calibrated to calculate a patient's arterial blood oxygen saturation. The method and apparatus of the present invention, as described hereinbelow, fulfills this need.